1. Field of Invention
The present invention relates generally to a control system for a vehicle with an electric motor drive system having a load-dependent current generating system (such as a fuel cell system) as the power source, and specifically to a method and system to identify the amount of Direct Current (DC) bus current needed to drive the electric motor based on driver demand, while controlling the actual amount of DC bus current drawn to match the amount of DC bus current made available from the load-dependent current generating system.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and pollutants from automobiles and other vehicles powered by an internal combustion engine (ICE) is well known. Vehicles powered by alternative energy sources are under development. For example, vehicles with battery powered electric traction motors are well known in the prior art. Other electric powered motor vehicles are also known having load-dependant current generators. An example of such a load-dependant system is a fuel cell system.
Fuel cells generate electrical power through an electrochemical reaction of a fuel and oxidant, such as hydrogen and oxygen. Water is the product of the electrochemical reaction in a fuel cell utilizing hydrogen and oxygen, a product that is easily disposed. See generally, U.S. Pat. No. 5,991,670 (Mufford et al. 1999).
The desirability of an electric motor powered vehicle is clear. Nevertheless, there remains substantial room for development of ways to control these vehicles. For example, in a typical battery powered electric vehicle, the electric motor drive system draws current from the battery as needed to power the electric motor. Battery controls are typically put into place to assure the electric motor drive system does not exceed various limits. Such limits can include the maximum current drawn from the battery, the minimum voltage the battery can be lowered while the motor draws current, the maximum current to charge the battery, and the maximum voltage the battery can be raised while charging.
In some control systems, constant values are used for these limits. For most vehicle operating conditions, the controls are not activated because these limits are rarely reached. In short, the electric motor drive system draws current from the battery in an xe2x80x9copen-loopxe2x80x9d fashion.
A fuel cell power source, or other load-dependent current generators, presents a more complex challenge. Here, the electric motor drive system can no longer draw current in the xe2x80x9copen-loopxe2x80x9d fashion described above. Controls associated with these types of systems are known in the prior art. Optimally, these controls should identify how much DC bus current the load (xe2x80x9cmotorxe2x80x9d) requires based on various driver demands given a specific command of Iq (or torque), and limit the load to the amount of current the fuel cell system (FCS) identifies is available. In other words, the controls must provide a xe2x80x9ccurrent commandxe2x80x9d to the FCS, that is used by the FCS to adjust its power output, resulting in an instantaneous xe2x80x9ccurrent available,xe2x80x9d that is provided back to the controls for matching the actual DC bus current drawn by the load.
Problems, or undesirable effects, result when the actual current drawn by the load does not draw the amount of xe2x80x9ccurrent available.xe2x80x9d First, if the actual DC bus current drawn by the load is more then what the FCS makes available, the resulting high voltage DC bus created by the FCS will drop in an undesired manner, that is, it may drop to lower than the FCS has anticipated, causing problems within the FCS. Second, if the actual DC bus current drawn by the load is less then what the FCS makes available, the FCS may begin to overheat and/or the overall system efficiency will decrease.
In summary, the fuel cell system is not xe2x80x9cload followingxe2x80x9d like a battery. The system requires an accurate method to determine DC bus current needs, and requires the load (the electric motor drive) to use the available DC bus current.
Simple, yet sub-optimal, control systems for fuel cell powered electric drive vehicles are known in the prior art. Sonntag, U.S. Pat. No. 5,877,600, discloses a method for determining the amount of DC bus current needed by the electric motor drive system, and provides this amount to the FCS to control the amount of power it generates. The only stated variable used in making load determination is accelerator pedal position. Additionally, this patent does not provide a method for the controller of the load to track, or follow, the amount of DC bus current available.
Sonntag also does not include a method to determine the maximum current available from the fuel cell system. Without this important determination, the electric motor drive system could potentially request more current than what the fuel cell system could possibly make available, leading to potential problems within the FCS.
Accordingly, the object of the present invention is to develop a more precise method and system for the controller of an electric motor (xe2x80x9cloadxe2x80x9d) to accurately determine the amount of DC bus current required from a load-dependent current generating system, such as a fuel cell, to operate the electric motor based on various vehicle inputs such as accelerator position, brake position, Key on/off, Gear selector position, and various component temperatures.
It is a further object of the present invention to use the monitored DC bus current drawn by any electric motor in a load-dependent current generating system to assure that the DC bus current drawn by the motor matches the amount of available DC bus current.
It is a further object of the present invention to use the monitored DC bus current drawn specifically by an AC induction electric motor in a load-dependent current generating system to assure that the DC bus current drawn matches the amount of available DC bus current.